A polypropylene film is broadly applied to electric use because it is more excellent in withstand voltage characteristics and electric properties such as dielectric loss than other plastic films. In particular, increase of demand as a dielectric material in the use of capacitors is remarkable. Recently, as various electric devices are made as a inverter formation, and further, as making a capacitor small-sized and large-capacity is accelerated, making a polypropylene film thinner is further required.
However, since a polypropylene film is a material poor in stiffness among plastic films used for capacitors, as its thickness is decreased, requirement of improvement of its handling ability and processing property has been increased. Therefore, various improvements of the handling ability and processing property have been proposed.
For example, a capacitor-use deposited polypropylene film is proposed, wherein a static friction coefficient at 80° C. between a non-deposited surface of a polypropylene film after metal deposition on the other surface of the film and a metal plate plated with chrome is 0.8 or less, and the content of an additive with a melting point of 130° C. or lower is 4,000 ppm by weight or less, and it is known to improve a property for the deposition by it (for example, JP-A-2-170406).
Further, known is a method for carrying out a surface treatment to a non-deposited surface at a wetting tension of 33 dyne/cm or less and giving the film a high-speed winding ability (for example, JP-A-58-16415). Furthermore, a polyester film is proposed wherein its F5 value is 11.0 kgf/mm2 or more and the content all particles in the film is in a range of 0.25-0.60 wt %, and it is known to improve the winding property and the slitting property (for example, JP-A-2000-204177).
Further, known is a method for suppressing reduction of withstand voltage and number of insulation defects of a film by controlling a difference Δd between a film thickness determined by micrometer method and a film thickness determined by weighing method in a range of 0.01-0.5 μm (for example, JP-A-10-156938). Furthermore, known is a method for controlling a ratio MMV/WMV of a micrometer thickness (MMV) to weight mean thickness (WMV) in a range of 1.2-1.6, thereby improving flowability between layers of insulation oil at the time of winding a film in an overlapping form (for example, JP-A-2001-118430).
However, in the capacitor-use deposited polypropylene film described in JP-A-2-170406, an effect for suppressing shift of winding in a capacitor element winding process is poor and insufficient. Further, in the method described in JP-A-58-16415, an effect for suppressing generation of wrinkles in a capacitor element winding process is insufficient, and in particular, it is remarkable in a thin film having a thickness of 4 μm or less. Furthermore, because the film described in JP-A-2000-204177 is a polyester film, it is difficult to apply the technology to a polypropylene film from the viewpoint such as a difference between polymer properties.
Further, any proposal is insufficient in withstand voltage characteristic which is one of final targets, especially, in a point for keeping an excellent withstand voltage characteristic for a thin polypropylene film with a thickness of 4 μm or less.
Further, in the method described in JP-A-10-156938, wrinkles generate particularly in a flattening press process of a capacitor element of a thin polypropylene film with a thickness of 4 μm or less, and therefore, it is insufficient in withstand voltage characteristics.
Furthermore, in the method described in JP-A-2001-118430, the porosity between film layers becomes too great, and the withstand voltage characteristics are greatly reduced in a capacitor into which insulation oil is not impregnated.
It could therefore be advantageous to provide a flat type capacitor-use polypropylene film which can suppress generation of shift and wrinkles in a capacitor element winding process or a flattening press process thereafter even if the film is a thin polypropylene film with a thickness of 4 μm or less, which is suitable to make a small-sized flat type capacitor with a large capacity of 500 μF or more for example, which keeps withstand voltage characteristics, in particular, self-recovering property that is one of the characteristics of a flat type capacitor, and which is suitable for use of a direct current rated voltage of 700V or higher, and a flat type capacitor using it.